1. Field of the Invention
The present invention relates to an offset press, more particularly, to a novel method and apparatus for dampening a plate cylinder thereof.
2. Description of the Related Art
In general, in a printing process utilizing an offset press, a water film is formed on part of a plate surface on which there is no image to be printed so that an ink imparted to the plate surface will not stick to the non-image area but will adhere only to an image-carrying area of the plate surface.
The water film must be thin and reproductive, and the film thickness must be varied in accordance with other conditions, such as printing speed, distribution of the image-carrying area and the non-image area, material of the plate, and property of the ink used.
The operation of a dampening unit for the offset press will be described below with reference to FIG. 1, in which part of an offset press provided with the most widely used dampening unit is schematically illustrated.
Water in a pan 1 is lifted by rotation of a water pan roller 2, and adhered to the surface thereof. A water ductor roller 3 is rotatably secured on one end of a lever 3a and the other end of the lever 3a is pivoted so as to swing between a first position where a surface of the water ductor roller 3 touches that of the water pan roller 2 and a second position where a surface of the water ductor roller 3 touches that of a chrome roller 4. The water ductor roller 3 intermittently transfers the water film formed on the pan roller 2 to the chrome roller 4 by this swing motion. Further, the chrome roller 4 is always in contact with a water form roller 5 disposed above the chrome roller 4, and reciprocates along the length thereof so that the water film carried on the surface of the chrome roller 4 is transferred onto the surface of the water form roller 5 as a uniform film. The water form roller 5 is always in contact with a plate 6a carried on the surface of a plate cylinder 6, and thus the water film on the water form roller 5 is transferred to a non-image area of the surface of the plate 6a.
Ink is fed from an ink fountain (not shown) to a vibration roller 7 and then to a pair of ink form rollers 8. An ink film formed on the ink form rollers 8 is then transferred to the surface of the plate 6a rotating integrally with the plate cylinder 6 while in contact with the surface of the ink form roller 8. During this transfer, the ink selectively adheres to the image carrying area of the plate 6a because the oily ink cannot adhere to the rest of the plate surface, i.e., the non-image area, because of the water film carried thereon.
The ink thus selectively adhered to the plate 6a is transferred to the surface of a constantly rotating rubber blanket cylinder 9 which is brought into contact with the plate 6a, and then is transferred to a paper 11 through a nip gap between the rubber blanket cylinder 9 and an impression cylinder 10 by the rotation of the cylinders 9, 10, and thus, a predetermined print out is obtained.
The conventional offset press provided with the above described dampening unit has the following drawbacks:
First, since the dampening unit uses a plurality of rollers for providing water to the surface of the plate 6a, it is very difficult to determine the most preferable conditions for forming a proper water film on the plate 6a. Accordingly, frequent trial-and-error operations, with the accompanying waste of time and paper, are necessary before the most favorable stable conditions have been established at the beginning of or a change-over during the printing operation.
Further, since the water form roller 5 is in constant rotation and in contact with the surface of the plate 6a, to which a residual ink film is still adhered, this ink film is liable to be transferred to the respective rollers of the dampening unit and thus contaminate the same. Accordingly, these rollers must be removed from the unit and cleaned when the color of the ink is to be changed. In addition, as the thickness of the ink film on the plate 6a becomes thicker, the ink is liable to be forced out of the predetermined image area due to the pressure of the water form roller 5 and so-called "ink threading" generates. This, of course, leads to a lowering of the printing quality.
Alternatives to the above dampening unit have been proposed. One such alternative is a "Dahlgren" type dampening unit. In this unit, a metal roller in stable contact with the surfaces of both the water pan roller 2 and the plate cylinder 6 is provided instead of the water ductor roller 3, and alcohol is added to the water to improve the adhesivity thereof to the roller. Another alternative is the "Beko" type dampening unit, which eliminates the water pan 1 and the water pan roller 2 but is provided with a rotor disc having a water conduit. The water is atomized to form a mist which-is jetted onto the surface of the water form roller 5 and transferred to the plate 6a via a series of rollers similar to those of the aforesaid unit. These alternatives are advantageous in that the water feeding rate is easily adjustable in accordance with the variance of the printing speed, even in a high speed operation. However, since in these apparatuses the water form roller is always in contact with the surface of the plate, the previously described drawback caused by the water form roller still remains.